agda/doc/release-notes/2.3.2.md

Release notes for Agda 2 version 2.3.2

Installation

• The Agda-executable package has been removed.

The executable is now provided as part of the Agda package.

• The Emacs mode no longer depends on haskell-mode or GHCi.

• Compilation of Emacs mode Lisp files.

You can now compile the Emacs mode Lisp files by running agda-mode compile. This command is run by make install.

Compilation can, in some cases, give a noticeable speedup.

WARNING: If you reinstall the Agda mode without recompiling the Emacs Lisp files, then Emacs may continue using the old, compiled files.

Pragmas and options

• The --without-K check now reconstructs constructor parameters.

New specification of --without-K:

If the flag is activated, then Agda only accepts certain case-splits. If the type of the variable to be split is D pars ixs, where D is a data (or record) type, pars stands for the parameters, and ixs the indices, then the following requirements must be satisfied:

• The indices ixs must be applications of constructors (or literals) to distinct variables. Constructors are usually not applied to parameters, but for the purposes of this check constructor parameters are treated as other arguments.

• These distinct variables must not be free in pars.

• Irrelevant arguments are printed as _ by default now. To turn on printing of irrelevant arguments, use option

  --show-irrelevant

• New: Pragma NO_TERMINATION_CHECK to switch off termination checker for individual function definitions and mutual blocks.

The pragma must precede a function definition or a mutual block. Examples (see test/Succeed/NoTerminationCheck.agda):

1. Skipping a single definition: before type signature.

    {-# NO_TERMINATION_CHECK #-}
    a : A
    a = a
   

2. Skipping a single definition: before first clause.

    b : A
    {-# NO_TERMINATION_CHECK #-}
    b = b
   

3. Skipping an old-style mutual block: Before mutual keyword.

    {-# NO_TERMINATION_CHECK #-}
    mutual
      c : A
      c = d
   
      d : A
      d = c
   

4. Skipping a new-style mutual block: Anywhere before a type signature or first function clause in the block

    i : A
    j : A
   
    i = j
    {-# NO_TERMINATION_CHECK #-}
    j = i
   

The pragma cannot be used in --safe mode.

Language

• Let binding record patterns

  record _×_ (A B : Set) : Set where
    constructor _,_
    field
      fst : A
      snd : B
  open _×_

  let (x , (y , z)) = t
  in  u

will now be interpreted as

  let x = fst t
      y = fst (snd t)
      z = snd (snd t)
  in  u

Note that the type of t needs to be inferable. If you need to provide a type signature, you can write the following:

  let a : ...
      a = t
      (x , (y , z)) = a
  in  u

• Pattern synonyms

A pattern synonym is a declaration that can be used on the left hand side (when pattern matching) as well as the right hand side (in expressions). For example:

  pattern z    = zero
  pattern ss x = suc (suc x)

  f : ℕ -> ℕ
  f z       = z
  f (suc z) = ss z
  f (ss n)  = n

Pattern synonyms are implemented by substitution on the abstract syntax, so definitions are scope-checked but not type-checked. They are particularly useful for universe constructions.

• Qualified mixfix operators

It is now possible to use a qualified mixfix operator by qualifying the first part of the name. For instance

  import Data.Nat as Nat
  import Data.Bool as Bool

  two = Bool.if true then 1 Nat.+ 1 else 0

• Sections [Issue #735]. Agda now parses anonymous modules as sections:

  module _ {a} (A : Set a) where

    data List : Set a where
      []  : List
      _∷_ : (x : A) (xs : List) → List

  module _ {a} {A : Set a} where

    _++_ : List A → List A → List A
    []       ++ ys = ys
    (x ∷ xs) ++ ys = x ∷ (xs ++ ys)

  test : List Nat
  test = (5 ∷ []) ++ (3 ∷ [])

In general, now the syntax

  module _ parameters where
    declarations

is accepted and has the same effect as

  private
    module M parameters where
      declarations
  open M public

for a fresh name M.

• Instantiating a module in an open import statement [Issue #481]. Now accepted:

  open import Path.Module args [using/hiding/renaming (...)]

This only brings the imported identifiers from Path.Module into scope, not the module itself! Consequently, the following is pointless, and raises an error:

    import Path.Module args [using/hiding/renaming (...)]

You can give a private name M to the instantiated module via

  import Path.Module args as M [using/hiding/renaming (...)]
  open import Path.Module args as M [using/hiding/renaming (...)]

Try to avoid as as part of the arguments. as is not a keyword; the following can be legal, although slightly obfuscated Agda code:

  open import as as as as as as

• Implicit module parameters can be given by name. E.g.

  open M {namedArg = bla}

This feature has been introduced in Agda 2.3.0 already.

• Multiple type signatures sharing a same type can now be written as a single type signature.

  one two : ℕ
  one = suc zero
  two = suc one

Goal and error display

• Meta-variables that were introduced by hidden argument arg are now printed as _arg_number instead of just _number. [Issue #526]

• Agda expands identifiers in anonymous modules when printing. Should make some goals nicer to read. [Issue #721]

• When a module identifier is ambiguous, Agda tells you if one of them is a data type module. [Issues #318, #705]

Type checking

• Improved coverage checker. The coverage checker splits on arguments that have constructor or literal pattern, committing to the left-most split that makes progress. Consider the lookup function for vectors:

  data Fin : Nat → Set where
    zero : {n : Nat} → Fin (suc n)
    suc  : {n : Nat} → Fin n → Fin (suc n)

  data Vec (A : Set) : Nat → Set where
    []  : Vec A zero
    _∷_ : {n : Nat} → A → Vec A n → Vec A (suc n)

  _!!_ : {A : Set}{n : Nat} → Vec A n → Fin n → A
  (x ∷ xs) !! zero  = x
  (x ∷ xs) !! suc i = xs !! i

In Agda up to 2.3.0, this definition is rejected unless we add an absurd clause

  [] !! ()

This is because the coverage checker committed on splitting on the vector argument, even though this inevitably lead to failed coverage, because a case for the empty vector [] is missing.

The improvement to the coverage checker consists on committing only on splits that have a chance of covering, since all possible constructor patterns are present. Thus, Agda will now split first on the Fin argument, since cases for both zero and suc are present. Then, it can split on the Vec argument, since the empty vector is already ruled out by instantiating n to a suc _.

• Instance arguments resolution will now consider candidates which still expect hidden arguments. For example:

  record Eq (A : Set) : Set where
    field eq : A → A → Bool

  open Eq {{...}}

  eqFin : {n : ℕ} → Eq (Fin n)
  eqFin = record { eq = primEqFin }

  testFin : Bool
  testFin = eq fin1 fin2

The type-checker will now resolve the instance argument of the eq function to eqFin {_}. This is only done for hidden arguments, not instance arguments, so that the instance search stays non-recursive.

• Constraint solving: Upgraded Miller patterns to record patterns. [Issue #456]

Agda now solves meta-variables that are applied to record patterns. A typical (but here, artificial) case is:

  record Sigma (A : Set)(B : A -> Set) : Set where
    constructor _,_
    field
      fst : A
      snd : B fst

  test : (A : Set)(B : A -> Set) ->
    let X : Sigma A B -> Sigma A B
        X = _
    in  (x : A)(y : B x) -> X (x , y) ≡ (x , y)
  test A B x y = refl

This yields a constraint of the form

  _X A B (x , y) := t[x,y]

(with t[x,y] = (x, y)) which is not a Miller pattern. However, Agda now solves this as

  _X A B z := t[fst z,snd z].

• Changed: solving recursive constraints. [Issue #585]

Until 2.3.0, Agda sometimes inferred values that did not pass the termination checker later, or would even make Agda loop. To prevent this, the occurs check now also looks into the definitions of the current mutual block, to avoid constructing recursive solutions. As a consequence, also terminating recursive solutions are no longer found automatically.

This effects a programming pattern where the recursively computed type of a recursive function is left to Agda to solve.

  mutual

    T : D -> Set
    T pattern1 = _
    T pattern2 = _

    f : (d : D) -> T d
    f pattern1 = rhs1
    f pattern2 = rhs2

This might no longer work from now on. See examples test/Fail/Issue585*.agda.

• Less eager introduction of implicit parameters. [Issue #679]

Until Agda 2.3.0, trailing hidden parameters were introduced eagerly on the left hand side of a definition. For instance, one could not write

  test : {A : Set} -> Set
  test = \ {A} -> A

because internally, the hidden argument {A : Set} was added to the left-hand side, yielding

  test {_} = \ {A} -> A

which raised a type error. Now, Agda only introduces the trailing implicit parameters it has to, in order to maintain uniform function arity. For instance, in

  test : Bool -> {A B C : Set} -> Set
  test true {A}      = A
  test false {B = B} = B

Agda will introduce parameters A and B in all clauses, but not C, resulting in

  test : Bool -> {A B C : Set} -> Set
  test true  {A} {_}     = A
  test false {_} {B = B} = B

Note that for checking where-clauses, still all hidden trailing parameters are in scope. For instance:

  id : {i : Level}{A : Set i} -> A -> A
  id = myId
    where myId : forall {A} -> A -> A
          myId x = x

To be able to fill in the meta variable _1 in

  myId : {A : Set _1} -> A -> A

the hidden parameter {i : Level} needs to be in scope.

As a result of this more lazy introduction of implicit parameters, the following code now passes.

  data Unit : Set where
    unit : Unit

  T : Unit → Set
  T unit = {u : Unit} → Unit

  test : (u : Unit) → T u
  test unit with unit
  ... | _ = λ {v} → v

Before, Agda would eagerly introduce the hidden parameter {v} as unnamed left-hand side parameter, leaving no way to refer to it.

The related Issue #655 has also been addressed. It is now possible to make `synonym' definitions

  name = expression

even when the type of expression begins with a hidden quantifier. Simple example:

  id2 = id

That resulted in unsolved metas until 2.3.0.

• Agda detects unused arguments and ignores them during equality checking. [Issue #691, solves also Issue #44]

Agda's polarity checker now assigns 'Nonvariant' to arguments that are not actually used (except for absurd matches). If f's first argument is Nonvariant, then f x is definitionally equal to f y regardless of x and y. It is similar to irrelevance, but does not require user annotation.

For instance, unused module parameters do no longer get in the way:

  module M (x : Bool) where

    not : Bool → Bool
    not true  = false
    not false = true

  open M true
  open M false renaming (not to not′)

  test : (y : Bool) → not y ≡ not′ y
  test y = refl

Matching against record or absurd patterns does not count as `use', so we get some form of proof irrelevance:

  data ⊥ : Set where
  record ⊤ : Set where
    constructor trivial

  data Bool : Set where
    true false : Bool

  True : Bool → Set
  True true  = ⊤
  True false = ⊥

  fun : (b : Bool) → True b → Bool
  fun true  trivial = true
  fun false ()

  test : (b : Bool) → (x y : True b) → fun b x ≡ fun b y
  test b x y = refl

More examples in test/Succeed/NonvariantPolarity.agda.

Phantom arguments: Parameters of record and data types are considered `used' even if they are not actually used. Consider:

  False : Nat → Set
  False zero    = ⊥
  False (suc n) = False n

  module Invariant where
    record Bla (n : Nat)(p : False n) : Set where

  module Nonvariant where
    Bla : (n : Nat) → False n → Set
    Bla n p = ⊤

Even though record Bla does not use its parameters n and p, they are considered as used, allowing "phantom type" techniques.

In contrast, the arguments of function Bla are recognized as unused. The following code type-checks if we open Invariant but leaves unsolved metas if we open Nonvariant.

  drop-suc : {n : Nat}{p : False n} → Bla (suc n) p → Bla n p
  drop-suc _ = _

  bla : (n : Nat) → {p : False n} → Bla n p → ⊥
  bla zero {()} b
  bla (suc n) b = bla n (drop-suc b)

If Bla is considered invariant, the hidden argument in the recursive call can be inferred to be p. If it is considered non-variant, then Bla n X = Bla n p does not entail X = p and the hidden argument remains unsolved. Since bla does not actually use its hidden argument, its value is not important and it could be searched for. Unfortunately, polarity analysis of bla happens only after type checking, thus, the information that bla is non-variant in p is not available yet when meta-variables are solved. (See test/Fail/BrokenInferenceDueToNonvariantPolarity.agda)

• Agda now expands simple definitions (one clause, terminating) to check whether a function is constructor headed. [Issue #747] For instance, the following now also works:

  MyPair : Set -> Set -> Set
  MyPair A B = Pair A B

  Vec : Set -> Nat -> Set
  Vec A zero    = Unit
  Vec A (suc n) = MyPair A (Vec A n)

Here, Unit and Pair are data or record types.

Compiler backends

• -Werror is now overridable.

To enable compilation of Haskell modules containing warnings, the -Werror flag for the MAlonzo backend has been made overridable. If, for example, --ghc-flag=-Wwarn is passed when compiling, one can get away with things like:

  data PartialBool : Set where
    true : PartialBool

  {-# COMPILED_DATA PartialBool Bool True #-}

The default behavior remains as it used to be and rejects the above program.

Tools

Emacs mode

• Asynchronous Emacs mode.

One can now use Emacs while a buffer is type-checked. If the buffer is edited while the type-checker runs, then syntax highlighting will not be updated when type-checking is complete.

• Interactive syntax highlighting.

The syntax highlighting is updated while a buffer is type-checked:

• At first the buffer is highlighted in a somewhat crude way (without go-to-definition information for overloaded constructors).

• If the highlighting level is "interactive", then the piece of code that is currently being type-checked is highlighted as such. (The default is "non-interactive".)

• When a mutual block has been type-checked it is highlighted properly (this highlighting includes warnings for potential non-termination).

The highlighting level can be controlled via the new configuration variable agda2-highlight-level.

• Multiple case-splits can now be performed in one go.

Consider the following example:

  _==_ : Bool → Bool → Bool
  b₁ == b₂ = {!!}

If you split on b₁ b₂, then you get the following code:

  _==_ : Bool → Bool → Bool
  true == true = {!!}
  true == false = {!!}
  false == true = {!!}
  false == false = {!!}

The order of the variables matters. Consider the following code:

  lookup : ∀ {a n} {A : Set a} → Vec A n → Fin n → A
  lookup xs i = {!!}

If you split on xs i, then you get the following code:

  lookup : ∀ {a n} {A : Set a} → Vec A n → Fin n → A
  lookup [] ()
  lookup (x ∷ xs) zero = {!!}
  lookup (x ∷ xs) (suc i) = {!!}

However, if you split on i xs, then you get the following code instead:

  lookup : ∀ {a n} {A : Set a} → Vec A n → Fin n → A
  lookup (x ∷ xs) zero = ?
  lookup (x ∷ xs) (suc i) = ?

This code is rejected by Agda 2.3.0, but accepted by 2.3.2 thanks to improved coverage checking (see above).

• The Emacs mode now presents information about which module is currently being type-checked.

• New global menu entry: Information about the character at point.

If this entry is selected, then information about the character at point is displayed, including (in many cases) information about how to type the character.

• Commenting/uncommenting the rest of the buffer.

One can now comment or uncomment the rest of the buffer by typing C-c C-x M-; or by selecting the menu entry Comment/uncomment the rest of the buffer".

• The Emacs mode now uses the Agda executable instead of GHCi.

The *ghci* buffer has been renamed to *agda2*.

A new configuration variable has been introduced: agda2-program-name, the name of the Agda executable (by default agda).

The variable agda2-ghci-options has been replaced by agda2-program-args: extra arguments given to the Agda executable (by default none).

If you want to limit Agda's memory consumption you can add some arguments to agda2-program-args, for instance +RTS -M1.5G -RTS.

• The Emacs mode no longer depends on haskell-mode.

Users who have customised certain haskell-mode variables (such as haskell-ghci-program-args) may want to update their configuration.

LaTeX-backend

An experimental LaTeX-backend which does precise highlighting a la the HTML-backend and code alignment a la lhs2TeX has been added.

Here is a sample input literate Agda file:

  \documentclass{article}

  \usepackage{agda}

  \begin{document}

  The following module declaration will be hidden in the output.

  \AgdaHide{
  \begin{code}
  module M where
  \end{code}
  }

  Two or more spaces can be used to make the backend align stuff.

  \begin{code}
  data ℕ : Set where
    zero  : ℕ
    suc   : ℕ → ℕ

  _+_ : ℕ → ℕ → ℕ
  zero   + n = n
  suc m  + n = suc (m + n)
  \end{code}

  \end{document}

To produce an output PDF issue the following commands:

  agda --latex -i . <file>.lagda
  pdflatex latex/<file>.tex

Only the top-most module is processed, like with lhs2tex and unlike with the HTML-backend. If you want to process imported modules you have to call agda --latex manually on each of those modules.

There are still issues related to formatting, see the bug tracker for more information:

https://code.google.com/p/agda/issues/detail?id=697

The default agda.sty might therefore change in backwards-incompatible ways, as work proceeds in trying to resolve those problems.

Implemented features:

• Two or more spaces can be used to force alignment of things, like with lhs2tex. See example above.

• The highlighting information produced by the type checker is used to generate the output. For example, the data declaration in the example above, produces:

  \AgdaKeyword{data} \AgdaDatatype{ℕ} \AgdaSymbol{:}
      \AgdaPrimitiveType{Set} \AgdaKeyword{where}

These LaTeX commands are defined in agda.sty (which is imported by \usepackage{agda}) and cause the highlighting.

• The LaTeX-backend checks if agda.sty is found by the LaTeX environment, if it isn't a default agda.sty is copied from Agda's data-dir into the working directory (and thus made available to the LaTeX environment).

If the default agda.sty isn't satisfactory (colors, fonts, spacing, etc) then the user can modify it and make put it somewhere where the LaTeX environment can find it. Hopefully most aspects should be modifiable via agda.sty rather than having to tweak the implementation.

• --latex-dir can be used to change the default output directory.